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United States Patent |
6,139,629
|
Kisielowski
,   et al.
|
October 31, 2000
|
Group III-nitride thin films grown using MBE and bismuth
Abstract
The present invention comprises growing gallium nitride films in the
presence of bismuth using MBE at temperatures of about 1000 K or less. The
present invention further comprises the gallium nitride films fabricated
using the inventive fabrication method. The inventive films may be doped
with magnesium or other dopants. The gallium nitride films were grown on
sapphire substrates using a hollow anode Constricted Glow Discharge
nitrogen plasma source. When bismuth was used as a surfactant,
two-dimensional gallium nitride crystal sizes ranging between 10 .mu.m and
20 .mu.m were observed. This is 20 to 40 times larger than crystal sizes
observed when GaN films were grown under similar circumstances but without
bismuth. It is thought that the observed increase in crystal size is due
bismuth inducing an increased surface diffusion coefficient for gallium.
The calculated value of 4.7.times.10.sup.-7 cm.sup.2 /sec. reveals a
virtual substrate temperature of 1258 K which is 260 degrees higher than
the actual one.
Inventors:
|
Kisielowski; Christian K. (Peidmont, CA);
Rubin; Michael (Berkeley, CA)
|
Assignee:
|
The Regents of the University of California (Oakland, CA)
|
Appl. No.:
|
055064 |
Filed:
|
April 3, 1998 |
Current U.S. Class: |
117/105; 117/108; 117/952 |
Intern'l Class: |
C30B 023/06 |
Field of Search: |
117/105,108,952
|
References Cited
U.S. Patent Documents
5657335 | Aug., 1997 | Rubin et al. | 372/44.
|
5725674 | Mar., 1998 | Moustakas et al. | 118/715.
|
5767581 | Jun., 1998 | Nakamura et al. | 257/749.
|
5804839 | Sep., 1998 | Hanaoka et al. | 257/123.
|
5843227 | Dec., 1998 | Kimura et al. | 117/101.
|
5888886 | Mar., 1999 | Sverdlov et al. | 438/505.
|
Primary Examiner: Kunemund; Robert
Attorney, Agent or Firm: Coudert Brothers
Goverment Interests
This invention was made with U.S. Government support under Contract No.
DE-AC03-76SF00098 between the U.S. Department of Energy and the University
of California for the operation of Lawrence Berkeley National Laboratory.
The U.S. Government may have certain rights in this invention.
Parent Case Text
This invention is disclosed in provisional application Ser. No. 60/043,042,
filed, Apr. 3, 1997, incorporated herein by reference, and this
application claims benefit of that provisional application.
Claims
We claim:
1. A crystaline, thin, group III-N film made by the method of,
a) providing a molecular beam epitaxy instrument;
b) cleaning a substrate and placing it in the instrument;
c) heating the substrate;
d) exposing the substrate to activated nitrogen;
e) depositing a III-N buffer layer on the substrate;
f) depositing bismuth on the substrate; and
g) simultaneously depositing at least one group III element and nitrogen on
the substrate.
2. The product of claim 1 wherein the at least one group III element is
chosen from the group consisting of gallium, aluminum, and indium, and
boron.
3. The product of claim 2 wherein more than one group III element is
deposited simultaneously with nitrogen on the substrate.
4. The product of claim 1 wherein the partial pressure of bismuth is
equivalent to that produced by a 623 K source.
5. The product of claim 1 wherein the bismuth is deposited from a source
having a temperature between about 475 K and about 875 K.
6. The product of claim 1 wherein the bismuth is deposited from a source
having a temperature between about 525 K and about 775 K.
7. The product of claim 1 wherein the bismuth is deposited from a source
having a temperature between about 575 K and about 675 K.
8. A method for making GaN films using molecular beam epitaxy comprising,
a) cleaning a substrate;
b) heating a substrate;
c) exposing the substrate to activated nitrogen;
d) depositing a group III-nitrogen buffer layer on the substrate;
e) depositing bismuth on the substrate; and
f) simultaneously depositing at least one group III element, and nitrogen
on the substrate.
9. The method of claim 8 further comprising depositing a p-type dopant on
the substrate simultaneously with depositing the at least one group III
element and nitrogen.
10. The method of claim 8 wherein the bismuth is deposited from a source
having a temperature between about 475 K and about 875 K.
11. The method of claim 10 wherein the bismuth is deposited from a source
having a temperature between about 525 K and about 775 K.
12. The method of claim 11 wherein the bismuth is deposited from a source
having a temperature between about 575 K and about 675 K.
13. The method of claim 8 wherein the bismuth is deposited from a source
having a temperature between about 800 K and about 1500 K.
14. The method of claim 13 wherein the gallium is deposited from a source
having a temperature between about 1000 K and about 1300 K.
15. The method of claim 8 wherein the magnesium is deposited from a source
having a temperature between about 400 K and about 700 K.
16. The method of claim 15 wherein the magnesium is deposited from a source
having a temperature between about 500 K and about 600 K.
17. The method of claim 8 wherein the nitrogen is deposited using a flux
rate between about 0.5 sccm and about 200 sccm.
18. The method of claim 17 wherein the nitrogen is deposited using a flux
rate between about 1 sccm and about 10 sccm.
19. The method of claim 8 wherein the substrate temperature is between
about 800 K and about 1100 K during crystal growth.
20. The method of claim 19 wherein the substrate temperature is between
about 900 K and about 1000 K during crystal growth.
21. The method of claim 8 wherein the at least one group III element is
chosen from the group consisting of gallium, aluminum, and indium, and
boron.
22. The method of claim 8 wherein more than one group III element is
deposited simultaneously with nitrogen on the substrate.
23. The method of claim 22 wherein two group III elements, gallium and
aluminum, are deposited on the substrate.
24. The method of claim 22 wherein two group III elements, gallium and
indium, are deposited on the substrate.
Description
I. BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to group III-nitride crystal growth and
more specifically to gallium nitride crystal growth.
2. Description of Related Art
Because they can emit light from the ultraviolet to the visible spectral
range, group-III nitrides (III-N) are the most promising materials for
optoelectronic light sources (S. Nakamura, et al., Jpn. J. Appl. Phys. 34,
L797 (1995); S. Nakamura, et al. Jpn. J. Appl. Phys 35, L217 (1996)). They
also hold promise for high frequency power devices (M. A. Kahn, et al.
Appl. Phys. Lett. 63, 1214 (1993). Usually, III-N films, for example GaN,
are grown at temperatures that are below half the melting point
temperature to prevent chemical decomposition during the growth process.
In addition to semiconductor materials, III-N films also comprise ceramics
which have many uses when deposited as thin films. This is described in,
"A study of the physical properties and electrochemical behavior of AlN
films" F. Vacandio, Y. Massiani, P. Gravier, Surface Coating and
Technology 92, 1997, 221.
Two primary methods are currently used to grow III-N films such as, for
example, GaN films: Metal Organic Chemical Vapor Deposition (MOCVD) and
Molecular Beam Epitaxy (MBE). MOCVD is performed at high temperatures,
about 1300 K, which leads to relatively high surface diffusion of
ad-atoms. Each atom that is newly deposited on a substrate surface is
referred to as an "ad-atom". Because there is a direct relationship
between surface diffusion and formation of large crystal grains, MOCVD has
in the past resulted in larger grain formation than MBE. Conventional
MOCVD yields grain diameters in the range between about 40 .mu.m to about
50 .mu.m while conventional MBE yields grain diameters between about 0.5
.mu.m and about 1 .mu.m. Large grain size is desirable because it
decreases the boundaries that charge carriers encounter, which in turn
increases carrier mobility. A full discussion of the importance of carrier
mobility and grain size can be found in "Semiconductor Devices, Physics
and Technology" S. M. Sze, New York Wiley, 1985. Ideally the entire thin
film grown would be of a single crystal. Instead, a number of crystals are
formed which grow into one another to form the thin film. It is important
to minimize the discontinuities at the boundaries between these crystal
growing regions and this is done both by growing larger crystal regions so
that there are fewer boundaries and by maintaining high degree of common
crystal lattice orientation between the crystal regions. When the lattice
orientation is maintained sufficiently, these crystal regions are referred
to as `grains` in the larger crystal thin film.
Although the MOCVD process results in large grain sizes, there are several
drawbacks to the process. Suitable metal organic precursors must be
available to create and dope the desired III-N thin film. The restriction
in starting materials limits the types of end product that can be formed.
In addition, the high temperatures at which MOCVD films are formed are
close enough to thermal equilibrium to limit variation in the percent of
various materials that make up the final film.
In contrast, the MBE growth process typically takes place at about 1000 K.
Forming a film at temperatures that are farther away from thermal
equilibrium of the III-N starting materials, allows higher concentrations
of dopant to be incorporated into the final structure. In addition, the
lower the film formation temperature is, the more possible it is to
manipulate the concentration of component parts of the final film.
Ad-atoms having less thermal energy are less likely to exclude a
substitute atom from the forming lattice. Thus, forming a film at a lower
temperature can help to increase doping levels or to grow high quality
structures comprising added elements such as aluminum and/or indium. For
example, using the relatively low crystal growth temperatures of MBE,
compounds such as Ga.sub.X Al.sub.1-X N and Ga.sub.Y In.sub.1-Y N can be
formed, where x represents the relative percent Ga with respect to Al and
y represents the relative percent Ga with respect to In. It can be seen
that when x=0, the first compound becomes aluminum nitride (AlN), and when
y=0, the second compound becomes indium nitride (InN). Alternatively, when
x and y=l, the compounds become GaN. The drawback to MBE films grown at
relatively low temperatures, for example about 30% to about 40% of a
compound's melting point temperature (about 1000 K for GaN), is that the
MBE films frequently exhibit a three-dimensional, instead of a
two-dimensional, crystal growth and grain sizes that are limited by
temperature and by strain. Films made using conventional MBE technology
therefore have exhibited reduced carrier mobility. Excess strain
originating from mismatched substrate and film lattice structures can be
optimized by growth of suitable buffer layers on the substrate before a
III-N film is grown. But it is not possible to increase the temperature at
which MBE takes place because the differential vapor pressures of the
nitrogen atoms alters the film composition under the vacuum in which MBE
takes place.
It would be very desirable to have a method of growing large
two-dimensional crystals of III-N compounds at low temperatures, utilizing
flexible MBE technology.
II. SUMMARY OF THE INVENTION
It is an object of this invention to, at relatively low temperatures, grow
III-N films having large two-dimensional crystals. It is a further object
of the present invention to use Molecular Beam Epitaxy (MBE) to grow large
two-dimensional films of III-N material.
The present invention comprises a two-dimensional crystalline film formed
by molecular beam epitaxy comprising, a plurality of two-dimensional group
III-nitride crystal grains having diameters between about 3 .mu.m and
about 40 .mu.m, wherein each crystal lattice structure coalesces between
the grains to form a continuous crystal; and a concentration of p-type
carriers that exceeds that currently possible by metal organic chemical
vapor deposition.
The present invention further comprises a crystaline, thin, group III-N
film made by the method of, a) providing a molecular beam epitaxy
instrument; b) cleaning a substrate and placing it in the instrument; c)
heating the substrate; d) exposing the substrate to activated nitrogen; e)
depositing a III-N buffer layer on the substrate; f) depositing bismuth on
the substrate; and g) simultaneously depositing at least one group III
element and nitrogen on the substrate.
III. SUMMARY DESCRIPTION OF THE DRAWINGS
FIG. 1: shows the surface diffusion coefficients as a function of the
reciprocal temperature for a sample GaN film grown with Bi as a surfactant
(solid circle) in comparison with GaN films grown without Bi as a
surfactant (open circles).
IV. DETAILED DESCRIPTION OF THE INVENTION
The present invention comprises a two-dimensional crystalline film formed
by molecular beam epitaxy comprising, a plurality of two-dimensional group
III-nitride crystal grains having diameters between about 3 .mu.m and
about 40 .mu.m, wherein each crystal lattice structure coalesces between
the grains to form a continuous crystal; and a concentration of p-type
carriers that exceeds that currently possible by metal organic chemical
vapor deposition.
The present invention further comprises a crystaline, thin, group III-N
film made by the method of, a) providing a molecular beam epitaxy
instrument; b) cleaning a substrate and placing it in the instrument; c)
heating the substrate; d) exposing the substrate to activated nitrogen; e)
depositing a III-N buffer layer on the substrate; f) depositing bismuth on
the substrate; and g) simultaneously depositing at least one group III
element and nitrogen on the substrate.
When atoms are deposited on a substrate surface during Molecular Beam
Epitaxy (MBE) they may stick on to the surface and not move along it. This
type of deposition results in an amorphous structure. Alternatively, they
may move, or diffuse, along the two dimensional surface of the substrate
before stopping. If they diffuse until they encounter another atom or
crystal structure they will bind into the crystal lattice and increase the
two dimensional size of the oriented crystalline film. Thus, increasing
diffusion lengths of newly deposited atoms increases the two-dimensional
size of the crystals. It would be ideal, for purposes of creating
optoelectronic films, if each deposited two dimensional film was a single
crystal. In the case of a gallium nitride film (GaN), that would mean that
each gallium would diffuse until it bound to a nitrogen in a suitable
lattice site and each nitrogen would diffuse until it bound to a gallium
in a suitable lattice site. Typically surface diffusion length is
increased by increasing the temperature of the process.
Differential evaporation of nitrogen during high temperature MBE has been
minimized by using large flux nitrogen sources. For example, a Constricted
Glow Discharges (CGD) plasma source, available at the Lawrence Berkeley
National Laboratory in Berkeley, California, was used to achieve a growth
rate of 1.4 .mu.m/hour for GaN. When several CGD sources are used, film
decomposition under high temperature vacuum can be overcome even at growth
temperatures greater than about 1073 K. However, the growth under high
nitrogen flux is largely vertical and the two-dimensional growth that
yields large grain diameter is not satisfactory. In contrast, the present
invention increases two-dimensional crystal growth, and thus grain
diameter, for films grown at about 1000 K.
The present invention comprises using bismuth (Bi) as a surfactant for
growth of III-N films by MBE. It is speculated that the presence of
bismuth on the substrate, increases the diffusion length of ad-atoms. Bi
is an especially good choice because Bi is isoelectronic with N but has a
much larger atomic size. The covalent radius of N is approximately 0.07 nm
while the covalent radius of Bi is approximately 0.15 nm. This difference
in radii makes it unlikely that large amounts of Bi becomes incorporated
into the GaN film during film growth. At the same time, however, Bi that
is located on the surface of the substrate interacts with Ga, causing it
to diffuse further. Other elements in the V column of the periodic table
could be used as surfactants, such as antimony (Sb) or arsenic (As). As
the size of the atom used as a surfactant decreases, it is more likely to
be substituted into the III-N lattice and affect the property of the film.
Because using Bi as a surfactant allows films having acceptable grain size
for optoelectric effects to be grown at lower temperatures than previously
possible, the composition of the film can also be altered. As the
temperature of film growth decreases, the film components become less
limited by solubility and the crystal lattice can accommodate stress and
elastic strain more easily so that a large percent of dopant can be
introduced. At the lower temperature, crystal atoms that are less
energetically favored, can be substituted more easily for the more
energetically favored ones. Furthermore, the solubility properties will
permit a greater variety of proportional constituency from substitute
group III elements. So, for example, some Al or In atoms may be
substituted for a certain number of Ga atoms. The amount of substitution
is determined by the properties desired in the resultant film. Typically
it is desirable to substitute between about 10% and about 40% of Al atoms
for Ga atoms; alternatively between about 5% and about 50% of In atoms are
typically substituted for Ga atoms. In some circumstances between about
20% and about 40% of In atoms are substituted for Ga atoms. In theory,
however, the substitution can range from 0 to 100%.
Also, doping occurs more easily when the growth temperature is further
below the thermal equilibrium temperature. The Mg dopant is larger than
Ga, so incorporation of Mg for Ga produces stress. When the growth, i.e.
substrate, temperature is higher, the lattice is more energetic in
excluding excess Mg atoms. It is also possible, when growing at lower
temperatures, to introduce secondary dopant atoms such as beryllium (Be)
which is smaller than both Ga and Mg. The Be then acts both as a p-type
dopant and lowers the average stress on the lattice introduced by the Mg
p-type dopant. Typically, about 10 times more dopant can be incorproated
into a film grown by MBE than one grown by MOCVD.
The inventive material is made by coevaporating at least one group III
element, and Bi in the presence of activated N onto a clean substrate.
Activated nitrogen is molecular or atomic nitrogen, or nitrogen ions, in
which some of the electrons are excited. Alternatively, Magnesium (Mg)
dopant is also coevaporated onto the substrate. The concentration of Bi,
Ga, and Mg is determined by controlling the temperature of the evaporation
source, which in turn determines the partial pressure of the element in
the MBE chamber. Thus it is common to cite the concentration of these
components in terms of their source temperatures. A variety of Bi
concentrations can be used, ranging from about 475 K to about 875 K.
Better results were observed in the range of Bi concentrations resulting
from a source temperature between about 525 K and about 775 K. The best
results were obtained using concentrations between about 575 K and about
675 K.
The source temperature of Ga can vary between about 1000 K and about 1300
K. Similarly the source temperature of Mg can vary between about 400 K and
about 700 K. More preferably, the Mg source temperature is between about
500 K and about 600 K.
Nitrogen flux is measured in standard cubic centimeters (sccm). The
nitrogen flux is typically maintained between about 0.5 sccm and about 200
sccm. More preferably the nitrogen flux is between about 1 sccm and about
10 sccm.
EXAMPLE 1
Undoped GaN films grown in the presence of Bi.
Substrates were prepared for deposition of the inventive optoelectronic GaN
film by cleaning steps that followed a standard cleaning process. The
substrates were degreased by boiling in acetone and ethyl alcohol for 5
minutes each and blown dry with nitrogen. After degassing in the load lock
for 30 min at 500.degree. C., they were transferred into the growth
chamber. The substrates were then heated up to 700.degree. C. for thermal
desorption of surface contaminants. At this temperature, they were exposed
to activated nitrogen for 10 minutes. Subsequently, a thin,
low-temperature GaN buffer layer (.about.250 .ANG.) was deposited on the
substrate. Its particular thickness was determined and optimized to
minimize the strain (Kisielowski, C. et al., Phys. Rev BII 54, 17745,
1996); M. S. H. Leung et al. Mat. Res. Symp. Proc. 449, 1997, p 221 and H.
Fujii et al. Mat. Res. Symp. Proc. 449, 1997, p.227); these three
references incorporated herein by reference.
Finally, the main epitaxial layer was grown on the buffer layer for 4
hours. GaN layers were grown using a rebuilt Riber 1000 MBE system.
Knudsen cells were used to evaporate pure Ga (99.9999%), and Bi
(99.9999%). The activated nitrogen was produced by a hollow anode CGD
plasma source using pure nitrogen gas (99.9999%) along with a Millipore
nitrogen purifier. Details of the source design are given by A. Anders and
S. Anders, Plasma Sources Sci. Technol. 4, 571, 1995. A dc voltage was
used to generate a glow discharge that was constricted to an area in the
plasma chamber close to the gas exit. The pressure difference between the
plasma chamber and the MBE growth chamber extracted the activated nitrogen
species having energies around 5 eV. A liquid nitrogen cryoshroud was used
during growth to obtain a base pressure in the chamber of
.about.2.times.10.sup.-10 Torr. A thin titanium (Ti) layer on the back of
the 10.times.11 mm.sup.2 c-plane sapphire substrate absorbed the heat
radiated from the tungsten (W) filament heater. The temperature of the
substrate was monitored with a pyrometer.
Typical growth conditions were: Ga source temperature: 1210 K; nitrogen
flow rate: 5-80 sccm; buffer-layer growth-temperature: 773 K; main-layer
growth-temperature: 1000 K. The growth temperatures are controlled by
heating the substrate to the desired temperature. The Bi source
temperature was varied in the range of 523 K to 823 K. During the growth,
the nitrogen partial-pressure in the chamber was in the range of between
about 10.sup.-5 and about 10.sup.-2 Torr.
Atomic force microscopy (AFM) images were made of nominally undoped samples
grown without Bi and of samples grown with different concentrations of Bi
(T.sub.Bi =623 K to 823 K). The images depicted areas that were 2.times.2
.mu.m.sup.2. The sample grown without any Bismuth exhibited small crystals
with irregular boundaries. Experiments have shown that these crystals can
be disconnected (M. S. H. Leung, et al. Proceedings of Materials Research
Society (MRS) 1996 Fall Meeting), and this was confirmed in a drastically
reduced carrier mobility of 6 cm.sup.2 /Vs. The presence of a small amount
of Bi, i.e. from source temperatures between 475 K and 625 K results in
the crystals coalescing at their boundaries. Concomitantly, Hall mobility
of n-type carriers increases to 73 cm.sup.2 /Vs. The background n-type
carrier concentration was unchanged and exceeded 10.sup.18 cm.sup.-3. This
rather high n-type concentration was probably caused by oxygen
contamination of the growth chamber which was opened before these runs. A
further increase of the Bi temperature to 723 K and 823 K yields continued
enlargement of the two-dimensional crystal size. However, at these higher
Bi concentrations there was poorer coalescence at the crystal boundaries
and the Hall mobility decreased, as shown in Table I.
TABLE 1
______________________________________
Carrier mobility as a function of the Bismuth temperature in GaN
films grown with and without Bismuth surfactant
______________________________________
T.sub.Bi (K)
-- 623 723 823
.mu.(cm.sup.2 /Vs)
6 73 10 3
______________________________________
EXAMPLE 2
Mg-doped GaN films grown in the presence of Bi.
Films were prepared as described above, using, in addition, Knudsen cells
to evaporate Mg (99.99% pure).
Magnesium (Mg) doped GaN thin films grown in the presence of Bi (T.sub.Bi
=350.degree. C. and T.sub.Mg =280.degree. C.) exhibited crystal sizes that
compared well with those of the unintentionally doped n-type films
described above. Unexpectedly, the size of the surface features, which
corresponded to two-dimensional crystal size, increased significantly to
about 10 .mu.m in diameter when Bi surfactant was used from a 623 K
source. When the temperature was increased, The background impurity
concentration was kept low and the intrinsic n-doping did not exceed
10.sup.17 cm.sup.3.
Previous experiments (Fujii et al., Ibid. 1997) suggested that the feature
sizes that can be observed on the films relate to the size of oriented
crystals which form the GaN thin films. This crystal size limitation was
attributed to a temperature and strain dependence of the Ga surface
diffusion coefficient (H. Fujii et al. Mat Res. Soc Symp., Vol 449, 1997).
The Bi surfactant thus appears to alter the Ga surface diffusion
coefficient on the GaN (0001) faces, too. If we assume that surface
diffusion only occurs during the growth of a double layer of Ga and N
(thickness 0.26 nm), a surface diffusion length (D .tau..sub.0).sup.1/2
can be estimated where D is the surface diffusion coefficient and
.tau..sub.0 is the time that is required to grow the double layer. Since
both the growth rate and the crystal size can be extracted from the
experiments we can estimate Ga surface diffusion coefficients. They are
shown as a function of reciprocal temperature in FIG. 1. Values for
samples grown without surfactant are shown by open circles and are taken
from Fujii reference above; they depict the temperature and stress
dependence of the surface diffusion coefficient. Values for samples grown
with surfactant are shown by solid circles. It is seen that the
utilization of Bi as a surfactant leads to a increase of the diffusion
coefficient. A surface diffusion coefficient was calculated as described
above that, with out the presence of Bi would have been expected for a
growth temperature of 1258 K. In the presence of Bi that diffusion
coefficient of 10.sup.-6 cm.sup.2 /sec, was observed for a sample is grown
at only 1000 K. Thus, the use of a bismuth surfactant is beneficial in
several respects. Use of Bi as a surfactants promotes increased crystal
size in III-N thin films grown at a given temperature. Additionally, when
used in the correct concentrations, concentration, the Bi surfactant
promotes coalescence of crystal boundaries.
Because using Bi as a surfactant allows films having acceptable grain size
for optoelectric effects to be grown at lower temperatures than previously
possible, the composition of the films can also be altered. At the lower
temperature, less energetically favored crystal atoms can be substituted
more easily for the more energetically favored ones. Furthermore, the
solubility properties will permit a greater variety of proportional
constituency from substitute group III elements. So, for example, some Al
or In atoms may be substituted for a certain number of Ga atoms. There are
thin crystaline films having, for example, x equal to about 15 (Ga.sub.85
Al.sub.15 N), or y equal to about 50 (Ga.sub.50 In.sub.50 N) that cannot
be made using MOCVD and which have particularly interesting properties.
These inventive films can be made by MBE using Bismuth as a surfactant
during film growth.
Also, doping occurs more easily when the growth temperature is further
below the thermal equilibrium temperature. The Mg dopant is larger than
Ga, so incorporation of Mg for Ga produces stress. When the growth, i.e.
substrate, temperature is higher, the lattice is more energetic in
excluding excess Mg atoms. It is also possible, when growing at lower
temperatures, to introduce secondary dopant atoms such as beryllium (Be)
which is smaller than both Ga and Mg. The Be then acts both as a p-type
dopant and lowers the average stress on the lattice introduced by the Mg
p-type dopant.
EXAMPLE 3
Lateral Overgrowth of III-N films
Lateral overgrowth is a technique currently used in MOCVD-grown films to
obtain regular grain sizes and shapes having low defect densities like
dislocations or grain boundaries. For lateral overgrowth to work, the
lateral growth rate of the film must exceed the vertical growth rate.
A substrate such as a silicon, sapphire, silicon carbide, germanium,
gallium nitride, gallium arsenide, or other wafer is coated with, for
example, an oxide layer, or a photoresist layer having about a thickness
on the order of nanometers (nm). Grooves are etched in the layer. The
groove dimensions are about 1 .mu.m wide and the spacing between the
grooves is about twice the radius of the typical grain radius on an
ungrooved surface.
The III-N film is first grown to fill the grooves, then continued growth
takes place, overgrowing the grooves until the films growing out of each
groove meet and coalesce in the middle of the space between the grooves
and a continuous thin film is formed.
Previously it was not possible to perform lateral overgrowth techniques
using MBE-grown films because the grain sizes were too small to properly
overgrow the space between the grooves. MBE-grown films of the present
invention however form laterally overgrown crystal layers, that
incorporate the benefits of being grown farther away from thermal
equilibrium than the MOCVD films are. The inventive laterally overgrown
films incorporate more dopant and are not limited to high temperature
solubilities of the component atoms.
EXAMPLE 4
Quasi-ternary systems including ceramic coatings.
Group III nitrides include a number of important compounds in addition to
semiconductors. For example, MBE ceramic films can be grown using Bi as a
surfactant at temperatures that are low relative to the melting point of
the compound. At these low temperatures, the composition is not limited to
melting point solubility factors and the ceramic films can be designed to
have many different compositions. The manufacture parameters of ceramic
films by MBE is know to practicioners skilled in the art and is not the
subject of this invention. The addition of bismuth during the MBE growth
of the ceramic films is new to the present inventive method and results in
novel ceramic formulations as well as larger grains than conventionally
achieved at a given temperature without use of Bi.
High concentrations of Al, even 100% Al (that is, x=0) form hard ceramics,
having high melting points. AlN is particularly important because of its
high thermal conductivity. Thin coatings of AlN are beneficial for a
variant of engine parts and computer parts.
Thus, the invention provides III-N semiconducting and ceramic films that
are not limited in their composition to melting point solubility factors,
can accept high concentrations of dopants, and form acceptably large grain
sizes for optoelectronic components. GaN thin films that exhibit up to 40
.mu.m diameter grains sizes were grown at 1000 K. The grain sizes were
actively determined by engineering the strain in the layers and by using
surfactants. The result suggests that the surface diffusion coefficient
can be varied by more than 4 orders of magnitude at a growth temperature
as low as 1000 K. In addition, surfactants cause the crystals in GaN thin
films to coalescence. This influences the lateral Hall mobility of the
films.
The description of illustrative embodiments and best modes of the present
invention is not intended to limit the scope of the invention. Various
modifications, alternative constructions and equivalents may be employed
without departing from the true spirit and scope of the appended claims.
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